Flat strip slab system

ABSTRACT

The present invention relates to a method and means of building a reinforced concrete construction employing a flat strip slab system which require less material and reduce over-all cost of construction, number of skilled workmen and as a viable alternative to conventional beam slab frame for medium and large span buildings. The defined strip is not a beam but the slab of larger thickness varied from column strip to its next strip and that is again different form bounded main panel.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to reinforced concrete construction and finds particular utility when applied as a method and means of building “floor slabs”.

BACKGROUND OF THE INVENTION

Beam slab frame is the conventional and most widely used structural frame for all ranges of spans. Both adequate literature and design basis are available for structural design.

The system suffers from drawbacks as beams are weak in torsion, congestion of column beam junctions which are structurally critical. Recent earthquakes amply demonstrate the weakness of beam slab frames. Detailing for earthquake resistant constructions, though has specific guidelines do not ensure easy way of achieving congestion free column beam joints.

Flat plate slabs are developed as an alternative to beam slab system and have become popular due to ease of construction.

As its primary focus was to replace Conventional Beam-Slab system, optimization was not looked at. Moreover, flat plate analysis is done using empirical methods.

In comparison, flat slabs demand higher slab thickness and steel content and hence higher dead weight.

Flat plate slabs rest directly on top of the columns which support them, subjected to large bending moments and shearing forces, and the effect of these high shears and flexural stress can cause failure by “punching” of the slab at the column-slab junction. Sometimes a flat slab cannot provide sufficient resistance to the shear forces imposed on it by a supporting column. Due to inappropriate distribution of material this system offers less resistance against lateral loading.

Flat plate slabs are highly under utilized structurally due to uniform thickness across total area. Stress contours clearly depict this fact.

SUMMARY OF THE INVENTION

In view of the foregoing disadvantages inherent in the known methods of now present in the prior art, the present invention provides construction of “floor slabs” done using flat strip slabs of concrete in place of Beam-slab or flat plate slabs. The flat strip slab solution employed in the present invention is based on structural optimization which works varied slab thickness determined according to the spans and other design considerations thereof. In general, such system not only save on direct material costs for the frame and the supporting foundations but also provide knock-on benefits in terms of structural performance, reduced height of the structure and allied benefits. Casting of slab system is same as conventional systems and offers no complication what so ever.

In the present invention flat strip slab construction places no restrictions on the positioning of horizontal services and partitions and can eliminate false ceiling as system by itself forms elegant patterns. This can eliminate PoP corbelling which is a superficial element in false ceiling, provided for aesthetics. This superficial element has the danger of falling under earthquake and pose danger to occupants.

My invention resides not in any one of these features per se, but rather in the particular combination of all of them herein disclosed and claimed and it is distinguished from the prior art in the particular combination of all of its structures for the functions specified.

There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated.

There are, of course, additional features of the invention that will be described hereinafter and which will form the subject matter of the claims appended hereto. Those skilled in the art will appreciate that the conception, upon which this disclosure is based, may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.

Further, the purpose of the foregoing abstract is to enable the U.S. Patent and Trademark Office and the public generally, and especially the scientists, engineers and practitioners in the art who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of the technical disclosure of the application. The abstract is neither intended to define the invention of the application, which is measured by the claims, nor is it intended to be limiting as to the scope of the invention in any way.

It is therefore an object of the present invention to provide a new and improved “Flat Strip Slab System” method which has all the advantages of the prior art “Floor Slabs” and none of the disadvantages.

It is another object of the present invention to provide a new and improved “Flat Strip Slab System” method which may be easily and efficiently built and marketed.

It is a further object of the present invention to provide a new and improved “Flat Strip Slab System” method which is of a durable and reliable “Floor Slab” construction.

An even further object of the present invention is to provide a new and improved “Flat Strip Slab System” method which is susceptible of a low cost of manufacture with regard to materials, and which accordingly is then susceptible of low prices of sale to the consuming public, thereby making such “Floor Slab” method economically available to the buying public.

Still yet another object of the present invention is to provide a new and improved “Floor Slab” method which provides in the apparatuses and methods of the prior art some of the advantages thereof, while simultaneously overcoming some of the disadvantages normally associated therewith.

Still another object of the present invention is to provide a new and improved “Floor Slab” system wherein the same leads to efficient stress leveling and utilization of material which further leads to low concrete consumption.

These together with other objects of the invention, along with the various features of novelty which characterize the invention, are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and the specific objects attained by its uses, reference should be had to the accompanying drawings and descriptive matter in which there are illustrated preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from the description below and from the appended drawings, which are meant to illustrate and not to limit the invention, and wherein:

FIG. 1 is a schematic view of a flat strip slab system, constructed in accordance with the present invention;

FIG. 2A is an cross-section, showing the case 1 of the flat strip slab system;

FIG. 2B is an cross-section, showing the underside of the flat strip slab system in FIG. 1;

FIG. 2C is a cross-sectional schematic view, showing the case 3 proposed flat slab system.

FIG. 2D is a cross-sectional schematic view of case 4 conventional beam-slab system.

DETAILED DESCRIPTION OF THE INVENTION

As discussed in the background section above, in wide beam concept, analysis is done using beam as a linear element and not homogeneous with slab. Width of wide beam is appreciable and above approach leads to consumption of higher quantity of material (both concrete and steel) besides additional fictitious loads in the overlap zone of beam and slab.

Flat strip slab concept can be approximated as an intermediate case between beam-column frame and flat slab structure. Concept of a) Homogeneous elements for analysis, b). Stress leveling, and c). Multiple steps through optimization in slab is the main difference over wide beam concept. Homogeneous elements remove fictitious loads and hence overall economy. The basis of the system is to achieve more uniform distribution of stresses in the slab. This automatically leads to multiple steps with primary wide strip along columns. This strip cannot be regarded as wide beam but can be conceived to be slab stiffening.

Further the defined strip is not a beam but the slab of larger thickness. And the thickness is varied as per the structural requirement across the span of the structure. (In other words based on bending and shear requirements).

Hence there is a presence of transitional strip between column strip and middle strip. It is a logical phenomenon to vary the thickness of the slabs as per the flow of stresses. Shear reinforcement does not become critical in strips once the depth for shear is accounted for.

The flat strip slab system described herein is successfully used for commercial, institutional buildings of both government and private owners for spans varying from 6 mm to 18 m. Live loads ranged from 3 to 10 kN/sqm. System has been approved by Indian Institute of Technology, Chennai, India & Government Engineering College (Jawaharlal Nehru Technological University College of Engineering), Hyderabad, India.

For the purpose of the comparison for general commercial and the parameters considered are 4 KN/sqm of live load, 2 KN/sqm of floor finishes, 4.0 KN/sqm of equivalent wall load with M25 concrete grade and Fe 415 steel. The structure is considered to be in Earthquake zone II with stilt+5 commercial floors.

Further, an analysis of the frame using other alternatives is done and four cases are studied in detail as:

-   Case 1: Frame analysis using flat strip slab with 275 mm at column     strips and 150 mm at remaining areas with beams as shown -   Case 2: Frame analysis using flat strip slab with 275 mm at column     strips (1200 mm wide) and 200 mm adjoining column strips (0.75 m on     either side) and 150 mm for centrally bound areas. -   Case 3: Frame analysis using flat slab concept of 275 mm thickness     without capitals. -   Case 4: Frame analysis using beam and column members only and the     slab load is considered as floor load. Dead weight of 225 mm thick     slab is considered. -   Case 4 may not be ideal solution when large spans are involved as     the grid slab system is appropriate for the span. However,     equivalent dead load from slabs will remain the same and hence the     effect on columns.

The analysis is done using typical finite element mesh used for floor and the following parameters are considered for detailed comparison.

-   a) Stress patterns in slabs: Bending stress, shear stress patterns     for each type are studied. Case 1 and case 2 have shown more even     distribution of moments as compared to Case 3. This is an     advantageous situation as the stress leveling occurs. -   b) Forces in beams and slabs around selected columns: Member/Element     forces are compared for all the cases. For Case 4 where only beams     are used, shear forces is converted into shear stress. At each     column, maximum stress is considered. General load combination with     a load factor of 1.5 is considered for comparison.

Moments in beams for Case 4 are high of all cases. Accordingly, steel requirement will be high due to high (Mo/b*d²) factor. For instance, the factor works out to be 7.6 at column 17. Hence the beam size is not workable and requires revision. In the other cases the said factor is low. Though Case 3 has less (Mo/b*d²) in comparison to Case 1 and 2, its concrete consumption is high.

-   c) Column steel for selected columns: Columns are subjected to both     axial forces and bi-axial moments. Since the combination will govern     the design, column steel in place of forces is considered for     comparison. Since there are more than 10 load cases, comparison of     reinforcement is convenient therein.

Column reinforcement is compared for selected columns of 600 mm×600 mm at all levels. At higher levels steel requirement is more or less same. Column steel in Case 1, Case 2 and Case 4 are comparable as there is no appreciable variation. This indicates the fact that flat strip slabs do not consume more steel in columns.

-   d) Deflections: Flat strip slabs of Case 1 and Case 2 have     deflections within limits and comparable with other systems. Onsite     measurements for deflections have shown much less than estimated.     Since pre-camber in shuttering is provided, these deflections will     not be of major concern. Analysis of flat strip slabs with higher     slab loads indicated that the deflections do not increase     appreciably indicating considerable redistribution. -   e) Consumption of materials per floor: Case 1 and Case 2 results in     definite 20% of material savings per floor. 

1. The method of forming a reinforced concrete flat strip slab system comprises of wide, rectangular strips especially used for floor slabs & roofs supported on columns directly; plurality of steps is formed on the said system as a result of varied thickness of the slab as per the structural requirement across the span of the structure; slab may possibly divided into middle, column and transition strips and corresponding reinforcement is based on the corresponding moments.
 2. The method of reinforced concrete flat strip slab system as claimed in claim 1, further leads to efficient stress leveling and utilization of material.
 3. The method of reinforced concrete flat strip slab system as claimed in claim 1, further advantageous with its low concrete consumption compared to the other systems.
 4. The method of reinforced concrete flat strip slab system as claimed in claim 3, said flat strip slab element has a strips of varying width and are suitable for medium to large spans.
 5. The method of reinforced concrete flat strip slab system as claimed in claim 4, in which the elongate strips of concrete have a thickness from 150 to 275 mm or more and a minimum width of 300 and maximum as per spans demands.
 6. The method of reinforced concrete flat strip slab system as claimed in claim 1, in which the reinforcing steel is of high yield strength deformed bars.
 7. The method of reinforced concrete flat strip slab system as claimed in claim 4, wherein stress patterns in slabs, forces in beams and slabs around selected columns, column steel for selected columns, deflections, composition of materials per floor are considered to determine economical, elegant alternative to the conventional reinforced concrete slab systems
 8. The method of reinforced concrete flat strip slab system as claimed in claim 5, wherein the columns are subjected to both axial forces and bi-axial moments as the combination will govern the design thereon.
 9. The method of reinforced concrete flat strip slab system as claimed in claim 5, is designed for all possible load cases of both gravity and lateral loads.
 10. The method of reinforced concrete flat strip slab system as claimed in claim 7, wherein efficient stress leveling can be acquired with more even distribution of bending stresses.
 11. The method of reinforced concrete flat strip slab system as claimed in claim 10, provide an economical and elegant alternative to the conventional building of slabs.
 12. The method of reinforced concrete flat strip slab system as claimed in claim 10, consumes less amount of concrete compared to the other conventional systems.
 13. The method of reinforced concrete flat strip slab system as claimed in claim 10, provide much higher specific surface as smaller diameter bars are used. 